1. Field of the Invention
The present invention is directed to a circuit for evaluating an input signal that is overlaid with a dc voltage part and, thereafter, for determining a data signal. The circuit includes (a) a signal input; (b) a signal processing means, including a filter, which forwards a part of the signal input; and (c) an evaluation means which compares the original signal from the signal input to the part of the signal forwarded from the signal processing means and determines a data signal.
2. Description of the Prior Art
Due to offset voltages, tolerances, etc., dc voltages are often superimposed on a signal. However, in order to optimally utilize the working range of the elements of a signal chain, it is also necessary to compensate for these dc voltages. Such compensation may be effected, for example, by determining the dc voltage of the signal part and then subtracting accordingly.
DC voltage parts are usually determined by a low-pass filtering process. An example which shows the necessity of such a low-pass filtering circuit is the synchronization in a TDMA system (Time Division Multiple Access). A TDMA system affords multiple access in a time-division multiplex method. Pursuant to such method, a transmitter transmits an alternating signal in what is referred to as a preamble. Due to the non-ideal properties in the transmission chain, additional dc voltage parts appear at the receiver output that must be compensated for in order to obtain optimum transmission quality. Since dc voltage parts or, more particularly, low-frequency parts are conceivable in normal data transmission, the dc voltage part within the preamble must be determined and then stored.
In a known circuit, the dc voltage part is determined by a low-pass filtering process. A compromise between speed and precision is made in view of the filter limit frequency and the time constant which is inversely proportional thereto. What is particularly problematical, however, is the determination of a switch point at which a switch is made from filtering to storing, since the system is not synchronized. A switch to storing can be made either as soon as possible and at the expense of precision or after waiting for the transition from preamble to the normal data. Given the employment of conventional linear filters, the latter also can ensue only with limited precision since the switchover can be recognized only when clearly tardy. In this case, the averaging of data becomes all the faultier the later recognition is made of the end of the preamble and the later that switching to storing occurs.
In a known approach to a solution, the delay is solved by using simple RC elements to average a delayed signal and not the current signal. The transfer response of such RC filters always has a monotonous character, i.e., the signal systematically runs away in one direction after the end of the preamble and a considerable hold error is caused accordingly. This error becomes smaller using filters of a higher order, but it does not completely disappear.